Contributing to 2015 individual 401k in 2016

[Kriegsspiel]

If I haven't contributed the full $18,000 to my i401k in 2015, can I finish maxing it out in 2016 as if it were an IRA, with money earned in 2015? What about if I funded it with the sale of a house I started flipping in 2015?

[terran]

Yes, you can do that. You need to have the solo 401k set up by the end of 2015, and make the effective date January 1, 2015.

You need to have schedule C income, so it would depend on how you're classifying the house flipping income -- it's possible that can count as a capital gain rather than earned income which would have it's own advantages, but I really don't know how that works.